Maraging steel article and method of manufacture

ABSTRACT

A fully dense, powder-metallurgy produced maraging steel alloy article of prealloyed powder for use as a tool for high temperature applications. The article in the as-produced condition having a hardness less than 40 HRC to provide machinability and thereafter the article upon maraging heat treatment having a hardness greater than 45 HRC. A method for producing this article comprises compacting prealloyed powder to produce a fully dense article having a hardness less than 40 HRC and thereafter maraging heat treating to a hardness greater than 45 HRC.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the manufacture of a maraging steel articlewith a specific composition using a powder metallurgy processing method.The steel as produced by practicing this invention, either in the AS-HIPcondition or HIPed and hot worked condition, is appropriate forapplications involving high temperatures or cyclic heating and cooling.The steel article of the invention has a hardness of less than 40 HRCafter manufacturing and after solution heat treating, allowing thearticle to be machined. However, after the manufacture of the articleand the subsequent maraging treatment, its hardness is greater than 45HRC.

The applications for the steel article of the invention includeprocessing of plastics or of liquid or hot solid metals, which includebut are not limited to mold dies for the casting of liquid metals, molddies for plastics, dies for forging other metals and dies for extruding.The cyclical heating and cooling of tools for these applicationscharacterize these applications. This cyclical heating and coolingcreate sufficient stresses in the tool to cause thermal fatiguecracking, also known as heat checking. Different applications cantolerate different amounts of heat checking. For some products thatrequire a high quality cosmetic appearance, the dies must be replacedafter very limited heat checking has occurred. For other products thatmay not require this high quality cosmetic appearance, the dies can beused even with severe heat checking. In all cases, the majority of dieseventually fail and are replaced due to thermal fatigue cracking.

Existing hot work tool steels can suffice for the products with lessstringent cosmetic requirements or shorter life time cycles. However,for product with a high cosmetic requirement, there is a need for a toolwith a longer useful service life to satisfy the demands of theproduction practice.

2. Prior Art

Tools are used in several applications involving the processing of hotmetal. This metal can be in the liquid form, as in die-casting, or inthe solid form, as in hot extrusion and hot forging. The useful life ofall these tool materials is typically limited by thermal fatiguecracking. That is, as the process proceeds, more thermal fatigue cracksinitiate on the surface of the tool, and existing thermal fatigue cracksgrow. The die is replaced when the extent of thermal fatigue crackingrenders the produced part as being of unacceptable quality. Requirementsof steel used for high temperature applications include:

The material must have the capability to be heat-treated to greater than45 HRC, which is the typical minimum working hardness for most tools ofthe prior art to maintain shape.

The material must also exhibit good high temperature strength. Fatiguecracking is related to the strength of the material. Therefore, a higherstrength is one factor that can improve the resistance to thermalfatigue cracking.

Due to the die being exposed to high temperatures, softening of the diematerial can occur. This softening of the material will also decreasethe strength of the material, making it more susceptible to thermalfatigue cracking. Therefore a tool material must exhibit good resistanceto softening, also known as temper resistance.

Many of the tools used in the above operations are taken out of servicedue to the presence of thermal fatigue cracks. Thermal fatigue crackinghas similarities to conventional fatigue cracking. However, in the caseof thermal fatigue cracking, the stresses are introduced in the tool bycyclic heating and cooling. Therefore, it is important that material forsuch a tool exhibit good resistance to thermal fatigue cracking.

The thermal expansion of the tool during the heating and cooling cycleintroduces stresses into the tool. Therefore, the material should haveas low a coefficient of thermal expansion as possible or at minimumlower than the current materials in use.

Many tools are coated for resistance to erosion. Therefore, the diematerial must be capable of being coated by PVD (physical vapordeposition) or other relevant coating.

Although some applications may use the invention in the AS-HIP (as hotisostatically pressed) condition, most applications will require the hotworking of the material into smaller sections suitable for the customer.Therefore, the material must have good hot workability.

Several materials are currently used the for hot work applications. TheH series tool steels were developed for these applications, with themost common being the 5Cr hot work tool steels. This includes the steelsknown in the United States as H13 and H11. The H13 steel class isnominally in weight percent 0.38 carbon, 5.25 chromium, 1.25 molybdenum,1.0 silicon and 1.0 vanadium. The H11 steel class is essentially thesame as the H13 class but with weight percent 0.5 vanadium. For moresevere applications, the H11 or H13 steel is typically processed usingelectro slag remelting (ESR) or vacuum arc remelting (VAR) methods.

Several variations of these 5 Cr tool steels have also been used. Amongthe most notable are H11 with lower silicon content for increasedtoughness. The other is a H11 with lower silicon and added molybdenumfor improved temper resistance. Table 1 shows the nominal chemistries ofsome standard and some non-standard commercially available tool steels.TABLE 1 Nominal Chemical Composition of Selected Standard and NonStandard Hot Work Tool Steels Alloy Designation C Si Mn Cr Mo V Co FeH10 0.32 0.25 0.30 3.00 2.80 0.50 — Bal. H10A 0.32 0.25 0.30 3.00 2.800.50 3.00 Bal. H11 0.40 1.00 0.25 5.30 1.60 0.40 — Bal. H13 0.40 1.000.40 5.30 1.40 1.00 — Bal. H19 0.45 0.40 0.40 4.50 3.00 2.00 4.50 Bal.Com. 1 0.36 0.20 0.50 5.25 1.65 0.50 — Bal. Com. 2 0.36 0.20 0.50 5.002.35 0.60 — Bal. Com. 3 0.36 0.20 0.40 5.20 1.95 0.60 — Bal. 1.2367 0.380.40 0.40 5.00 3.00 0.60 — Bal. Com. 4 0.38 0.20 0.25 5.00 3.00 0.60 —Bal.

Among other materials which have been used in the past for hot workapplication are maraging steels. Most of them contain approximately 18%nickel and some titanium and obtain their hardness by precipitation ofNi—Mo and Ni—Ti particles. Many of these steels are aged using arelatively low temperature, typically less than

1000° F. which can limit the usefulness of the material when exposed tohigher temperatures. Table 2 shows the nominal chemistries of somecommercially available maraging steels. TABLE 2 Nominal ChemicalComposition of Selected Maraging Steels Alloy C Si Mn Ni Cr Mo Co Cu TiAl B Com. 1 0.008 0.15 0.05 17.5 0.10 4.90 11.00 0.20 0.13 — 0.003 Com.2 0.02 0.04 0.03 18.5 0.05 4.80 7.50 — 0.40 0.10 0.003 Com. 3 0.02 0.050.03 18.5 0.10 4.90 9.00 — 0.60 0.10 0.003 Com. 4 0.02 — — 12.0 — 8.008.00 — 0.50 0.05 —

Some conventional maraging steels have been developed in the past withgood thermal fatigue resistance and strength, but when produced byconventional methods have exhibited poor hot workability duringprocessing from ingot stage to finished form. This poor hot workabilityresults in either a defective final product or an insufficient yield(less than 50%) from ingot stage to finished stage to render the productcommercially viable.

SUMMARY OF THE INVENTION

The invention provides a new powder metallurgy produced maraging steelalloy article to be used as a tool for high temperature applicationsthat satisfies the above-stated requirements. The article is fully denseand of prealloyed powder particles. TABLE 3 Chemistry Ranges for Alloyof Invention C Mn Si Cr Mo Ni Co S Broad 0.00-0.08 0.00-1.00 0.00-1.002.50-6.00  6.00-10.00 1.00-4.00  9.00-14.00 0.00-0.30 Range Preferred0.00-0.05  0.10-0.050 0.010-0.50  4.00-5.75 7.00-9.00 1.50-3.0010.00-13.00 0.005-0.05  Range More 0.01-0.04 0.20-0.40 0.15-0.404.70-5.30 7.50-8.50 1.70-2.30 10.75-12.00 0.01-0.03 Preferred Range

Hardening of the material is achieved by solution annealing and ageing,i.e. heating at a prescribed temperature for a prescribed length oftime. This allows small precipitate particles to form, which in turnharden the low carbon martensitic structure of the material.

In the following, the importance of the individual alloying elements andtheir mutual interaction will be explained. All percentages related tothe chemical composition in the specification and claims are in weightpercent.

Molybdenum is a key element in the strengthening of this maraging steel,as the precipitate responsible for hardening the alloy is Fe₂Mo. It isalso a key element in increasing the temper resistance of the alloy.Excessive quantities of molybdenum can allow the formation ofdetrimental delta ferrite.

Cobalt is required in a proper balance to prevent undesirable phases andto influence the aging process. Cobalt is an austenite former whilepreventing the formation of delta ferrite at high temperatures and has aminimal effect on the austenite to martensite transformationtemperature. Cobalt also lowers the solubility of molybdenum in themartensitic matrix, thus making molybdenum more available forprecipitation.

Chromium is desirable in some quantity for resistance to hightemperature oxidation. Chromium in excessive quantity can result in theformation of delta ferrite.

Nickel also provides some benefit to oxidation resistance and isbeneficial to mechanical properties. Excess nickel can cause theformation of austenite at typical service temperatures.

Carbon is not a critical element in the strengthening mechanism of thismaterial.

Silicon is not a critical element in the properties of the alloy.Silicon may be used for deoxidizing during melting. It is a strongferrite stabilizer.

Manganese is not critical for the properties of this alloy. It can beused to form manganese sulfide and therefore the content should beincreased with increasing quantities of sulfur for enhancedmachinability.

Sulfur may be present to promote machinability.

Vanadium, niobium, titanium, tungsten, zirconium, aluminum and otherstrong carbide and/or nitride formers are elements that are not desiredand therefore should not exist in amounts above incidental impuritylevels.

The alloy article of the invention is provided in the solution-annealedcondition, which is performed by heating the material between 1740° F.and 1925° F. Hardening by maraging is achieved by heating the materialbetween 1050° F. and 1360° F.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the comparison of an alloy specimen within thecomposition limits of the invention produced by powder metallurgy andone produced by ESR with respect to ductility;

FIG. 2 is a graph comparing the thermal fatigue resistance of a specimenin accordance with the invention and a specimen of H13 alloy; and

FIG. 3 is a graph comparing hardness of a specimen in accordance withthe invention and a specimen of H13 alloy.

PERFORMED EXPERIMENTS AND SPECIFIC EXAMPLES

Experiments were performed to determine various properties that wereconsidered important to the successful performance of the alloy articleof the invention. This included rapid strain tensile testing as ameasure of hot workability, thermal fatigue cracking, temper resistance,tensile testing at room temperature and at 1000° F., determination ofcoefficient of thermal expansion and coating trials.

The following is the steel composition of the invention and H13composition of the test specimens: Element Maraging Alloy ESR H13 C0.019 0.40 S 0.011 0.002 Mn 0.32 0.27 Si 0.27 1.05 Cr 4.92 5.46 Mo 7.871.22 V <0.005 0.91 Co 11.17 0.04 Ni 1.89 0.15 P 0.015 0.009 Al <0.0050.01 Nb <0.005 <0.01 Ti <0.005 <0.01 W 0.007 <0.01 O 0.011 0.0017 N0.023 0.005Rapid Strain Tensile Test

The rapid strain tensile testing was performed using the alloy articleof the invention produced by powder metallurgy and electro slag remeltedmaterial of the same composition. In rapid strain testing, the specimenswere heated by direct resistance heating. After achieving and equalizingat the desired test temperature, a load was applied to achieve a strainrate of 550 in/in/minute. This test is useful in simulating theconditions that exist during the hot working of the material.

Test temperatures were 1800° F., 1900° F., 2000° F., 2100° F., 2150° F.,2200° F. and 2250° F. FIG. 1 shows the reduction in area of the rapidstrain rate tensile test for the specimens produced of the alloy ofinvention and the ESR material of the same composition. This clearlyshows a substantial ductility advantage for the powder metallurgymaterial. The ductility of the ESR material was insufficient to permithot working.

The rapid strain tests also are in agreement with experience on fullsize trials. Two full size compacts of the powder metallurgy alloycomposition of the invention were produced and consolidated by hotisostatic pressing. Each compact was then processed to an intermediatesize and then to a final size by hot rolling. Neither compact exhibitedany hot working difficulties and the process yield was within the rangeof standard processing yield for other tool steels. By contrast, trialswith full size ingots produced by ESR or other conventional methodsexhibited poor hot workability during processing at the commercial steelmaking facility, resulting in process yields well below standard,including two heats that were scrapped entirely.

Thermal Fatigue Resistance

Another important characteristic of hot work tool steels is thermalfatigue resistance. There are several tests available to measure thermalfatigue cracking, although none of these tests are a standard method(e.g. ASTM). Some testing is performed by heating a specimen to a hightemperature using an induction coil for heating, then allowing thespecimen to cool. This is performed over a number of cycles, with thespecimen being evaluated periodically during the test. Another methodinvolves testing a specimen with an internal cooling cavity for coolingwater. This specimen is repeatedly immersed into a liquid aluminum bath.Again the cracking is rated periodically during the test.

The testing for the alloy of the invention was performed using a ½″square by 6″ long solid specimen produced by hot isostatic pressing andhot working. The test specimen can be tested simultaneously with up tofive other specimens during the same procedure. The other specimen forthis experiment was an ESR H13 material, which is the alloy mostfrequently used in aluminum die casting dies. The specimens were boltedto a holding plate affixed to the end of a mechanical arm which movedthe specimens through the various stages of the test cycle. The armimmersed the specimens into molten aluminum to a depth of approximately5 inches for 7 seconds. The specimens were then lifted out of the moltenaluminum, moved to a position above a tank of water and then immersedinto the water for 12 seconds. The specimens were then lifted out of thewater, and the arm moved to a position above the molten aluminum for 5seconds to dry the specimens. The cycle was then repeated.

During the trials, the specimens were periodically evaluated for thermalfatigue cracking, typically every 5,000 cycles. Two opposite faces ofthe specimens were cleaned using silicon carbide paper on a granitesurface plate. The four cleaned corners of each specimen were thenexamined under a stereo microscope at a magnification of 90×. To avoidend effects, the examinations were conducted in an area 1⅜″ long, andwhich was located about 1⅜″ from the bottom end of the specimens.

Each of the four corners was traversed along the 1⅜″ length and thenumber of cracks and their lengths were recorded. There are numerousways this data can be normalized, but experience with the test has shownlittle deviation in the ranking of the specimens. Therefore, the simpletotal number of cracks was divided by the number of corners (4) toobtain the number of cracks per corner. FIG. 2 is a graphicrepresentation of trial results of the powder metallurgy producedinvention specimen versus the ESR H13 steel specimen. As previouslydiscussed, thermal fatigue cracking is the most frequent cause of toolfailure. For this reason, it is believed that thermal fatigue testingprovides the most important indication of the improved performance ofthe alloy of invention.

Temper Resistance

A trial to determine the temper resistance of the alloy article of theinvention was also performed. Both the PM alloy specimen of theinvention and the H13 steel specimen were heat-treated to similarhardness levels, using typical heat-treat cycles for each material. Aninitial hardness was measured and recorded. Then the specimens wereplaced into a furnace at a temperature of 1200° F. One set of specimenswas removed after 50 hours at temperature and the hardness level testedand recorded. Another set of specimens was removed after 100 hours attemperature and the hardness level tested and recorded. FIG. 3 is agraphical representation of the hardness level as a function of holdtime at 1200° F. It can be seen that the alloy of the invention has asuperior temper resistance to H13 steel.

Tensile Properties

Table 4 shows the results of tensile testing of the PM alloy article ofthe invention versus results for ESR H13 steel. Specimens tested weremachined to a 0.250″ diameter with a 1.00″ gage length (4D). The resultsindicate that the alloy of invention has a higher yield and tensilestrength at both room temperature and at 1000° F. This higher strengthmakes the alloy article of the invention less susceptible to thermalfatigue cracking. TABLE 4 Tensile Properties Invention Maraging ESR H13Steel Article (47 HRC) (45 HRC) 72° F. UTS 261 206 YS 207 185 % EI 10 12RA 25 55 1000° F. UTS 161 145 YS 138 116 % EI 23 15 RA 62 75Coefficient of Thermal Expansion

Thermal expansion is an important factor, both in the resistance of atool to thermal fatigue cracking and in the final product quality of atool. In both cases, a smaller coefficient of thermal expansion isdesired. The significance of the lower coefficient of thermal expansionis that with less dimensional change, the tool will be subjected tolower thermal stresses than a material with a greater dimensionalchange. The lower stresses present will thus render the tool moreresistant to thermal fatigue cracking. The coefficient of thermalexpansion was determined by the thermal dilatometric analysis (TDA)method. The coefficient of thermal expansion for the PM alloy article ofthe invention was determined to be 6.6×10⁻⁶ in./in./° F. over thetemperature range of 72° F. to 1110° F. The ESR H13 die steel had acoefficient of 7.3×10⁻⁶ in./in./° F. over the temperature range of 72°F. to 1110° F.

Field Coating Trials

Field trials have shown the PM invention alloy article is easily coatedwith either a physical vapor deposition (PVD) process or chemical vapordeposition (CVD) which employs a higher temperature than the PVDprocess. The alloy article of the invention was coated with TiN, TiAlNand CrN PVD coatings. The coatings were deposited at a high depositionrate at a temperature range of 750-850° F. for both the article of theinvention and ESR H13 steel. Unlike many other maraging steels, thistemperature is well below the aging temperature for the alloy article ofthe invention.

Similarly, the coating was deposited using a chemical vapor depositionprocess on both the alloy article of the invention and conventional toolsteel material. Conventional tool steels are not well suited for CVD, asthe coating process typically takes place at a temperature above thecritical temperature of these alloys. The advantage provided by thearticle of the invention is that the CVD process results in the requiredheat treatment, namely solution annealing. After coating, the inventionarticle requires only a single aging treatment. The nature of themaraging process is such that the dimensional changes of the tool arevery minimal, allowing for good adherence of the coating to thesubstrate.

1. A fully dense, powder-metallurgy produced maraging steel alloyarticle of prealloyed powder for use as a tool for high temperatureapplications comprising, in weight percent: C 0.08 max., Mn 1.0 max., Si1.0 max., Cr 2.5-6.0, Mo 6.0-10.0, Ni 1.0-4.0, Co 9.0-14.0, sulfur up to0.3 and balance iron and incidental elements and impurities; saidarticle as-produced having a hardness of less than 40 HCR to providemachinability; and thereafter said article upon maraging heat treatmenthaving a hardness greater than 45 HRC.
 2. The article of claim 1,comprising, in weight percent, C 0.05 max., Mn 0.1 to 0.05, Si 0.01 to0.5, Cr 4 to 5.75, Mo 7 to 9, Ni 1.5 to 3, Co 10 to 13,S 0.005 to 0.05and balance iron and incidental elements and impurities.
 3. The alloyarticle of claim 1 comprising, in weight percent, C 0.01 to 0.04, Mn 0.2to 0.4, Si 0.15 to 0.4, Cr 4.7 to 5.3, Mo 7.5 to 8.5, Ni 1.7 to 2.3, Co10.75 to 12, S 0.01 to 0.03 and balance iron and incidental elements andimpurities.
 4. The article of claims 1, 2 or 3 in the solution-annealedcondition.
 5. The article of claims 1, 2 or 3, wherein said article isin the form of a die.
 6. The article of claims 1, 2 or 3, wherein saidarticle is in the form of a container for liquid metal.
 7. A method forproducing an article for use in processing of hot metal comprising:compacting prealloyed powder of a maraging steel to produce a fullydense article having a hardness of less than 40 HRC to providemachinability; thereafter maraging heat treating said article to achievea hardness greater than 45 HRC; and said prealloyed powder comprising,in weight percent, C 0.08 max., Mn 1.0 max., Si 1.0 max., Cr 2.5-6.0, Mo6.0-10.0, Ni 1.0-4.0, Co 9.0-14.0, sulfur up to 0.3 and balance iron andincidental elements and impurities.
 8. The method of claim 7, whereinsaid prealloyed powder comprises, in weight percent, C 0.05 max., Mn 0.1to 0.05, Si 0.01 to 0.5, Cr 4 to 5.75, Mo 7 to 9, Ni 1.5 to 3, Co 10 to13, S 0.005 to 0.05 and balance iron and incidental elements andimpurities.
 9. The method of claim 7, wherein said prealloyed powdercomprises, in weight percent, C 0.01 to 0.04, Mn 0.2 to 0.4, Si 0.15 to0.4, Cr 4.7 to 5.3, Mo 7.5 to 8.5, Ni 1.7 to 2.3, Co 10.75 to 12, S 0.01to 0.03 and balance iron and incidental elements and impurities.
 10. Themethod of claim 7, 8 or 9, wherein maraging heat treatment is performedat temperatures within a range of 540 to 700 degrees C.